1. Field of Use
This invention relates generally to holographic mirrors and apparatuses and methods for recording such mirrors. More specifically, the present invention relates to high volume production of Lippman holographic mirrors.
2. Description of the Prior Art
Significant progress in high-resolution phase holographic materials such as dichromated gelatin (DCG), PVA-based polymers, PMMA-based polymers and DCG/polymer grafts, has stimulated interest in several attractive applications of high efficiency Bragg holography, including holographic notch filters, Lippman holographic mirrors, holographic optical elements (HOE), display holograms, holographic gradings, and, recently, VLSI holoplanar interconnects.TM..
In all of these applications, very intense (&gt;5W) and stable continuous power argon laser sources may be used. Bragg wavelength shift (0.3-3 um), tunability of Bragg bandwidth (5 nm-300 nm), and variable refractive index modulation (0.0001-0.1) are achieved by suitable coating, exposing, and processing. All holograms regardless of their application are coated, exposed, and processed during manufacture.
Advances in each of these manufacturing processes will be important to the further advancement of and mass production of high resolution phase holographic materials in all applications.
Lippman holographic mirrors are just one of the many applications for such high resolution holographic materials but an extremely important and useful application that affords many important advantages over other types of holograms. Lippman holographic mirrors are holograms whose Bragg surfaces are parallel to one or more of the surfaces of the hologram. Referring to FIG. 5(a) a Lippman hologram is shown. The Bragg planes are parallel to the top and bottom surfaces of the holographic volume. Referring to FIG. 5(b), a non-Lippman hologram is depicted It can be seen that the Bragg surfaces are parallel to none of the surfaces of the volume.
Lippman holograms are extremely useful in many applications for several reasons, one primary one being that Lippman holograms do not suffer from chromatic (wavelength) dispersion. Referring to Eq. 1: ##EQU1## which shows the relationship between .alpha., the angle of incidence, .beta., the angle of diffraction, .lambda., the optical wavelength, and .LAMBDA..sub.//, the grating constant horizontal component, it is seen that the angles .beta. and .alpha. are related to the ratio of the optical wavelength to the grating constant
Referring now to FIG. 6, the angles .alpha. and .beta. are shown and the x or horizontal axis schematically represents a hologram. It can be seen that the angle .alpha. is the angle of incidence of the light wave upon the hologram and is defined as the angle between the axis perpendicular to the hologram and the incident light wave. Likewise it can be seen that the angle .beta. is defined as the angle between the axis perpendicular to the hologram and the exiting angle of the light wave.
Referring back to FIG. 5, the grating constant, .LAMBDA., is the distance between Bragg planes in the holograms. In FIG. 5(a), the grating constant .LAMBDA. is entirely in the vertical direction and is the distance between the horizontally oriented Bragg planes in that figure. Note that the horizontal component of the grating constant, .LAMBDA..sub.//, equals infinity. Referring again to FIG. 5(b) the grating constant .LAMBDA. is the distance between the Bragg planes oriented from the lower left to upper right. Also shown in FIG. 5(b) is the grating constant in the horizontal direction, .LAMBDA..sub.//. In this case the horizontal component of the grating constant has a finite value equal to the projection of the grating constant .LAMBDA. on the horizontal axis.
Differentiating Eq. 1 yields the following relationship between .beta., the change in the optical wavelength, .DELTA..lambda., and the horizontal component of the grating constant .LAMBDA..sub.//. ##EQU2## Using Eqs. 1 and 2, we get ##EQU3## where sin.beta. is defined by Eq. 1. Eq. 3 thus yields the relationship between changes in the diffraction angle .beta. with respect to changes in the optical wavelength .lambda. and the grating constant in the horizontal direction, .LAMBDA..sub.//. It can be seen from equation 3 that if .LAMBDA..sub.// is reduced. .DELTA..beta. is increased. For .LAMBDA..sub.// =.infin. (which is true for the Lippman case as shown in FIG. 5(a)), then .DELTA..beta.=0 and dispersion does not exist. These equations show that Lippman holographic mirrors have no chromatic dispersion.
Chromatic dispersion should be avoided in holograms in most applications such as protective visors or window films because any kind of dispersion (a change in .beta.) causes a rainbow effect. The rainbow effect, although aesthetic, is not satisfactory for most holographic mirror applications. Although the rainbow effect occurs most strongly for the transmission case, even reflection holograms have some minor degree of rainbow effect in transmission due to the presence of some surface relief in the transmission hologram. Referring to FIG. 7, a non-Lippman hologram is shown. It can be seen on the lower edge of the hologram that a certain degree of surface relief corresponding to the Bragg surfaces within the hologram is present. This surface relief can cause unwanted rainbow effects in transmission which obstruct see-through.
Lippman holographic mirrors can be made either narrow or broad band. Lippman holographic mirrors can be made broad band by introducing vertical non-uniformities into the hologram. As shown in FIG. 4, depicting a graph whose vertical access is distance in the Z direction and whose horizontal axis is distance in the X direction, a non-uniform Lippman holographic mirror is shown. The non-uniformity in FIG. 4(a) is the decrease in spacing between the Bragg planes in the Z direction. It can be seen that the Bragg planes become closer together, and thus the grating constant .LAMBDA. becomes smaller, as Z increases. Referring to FIG. 4(b), which is a graph comparing distance in the Z direction to the grating constant .LAMBDA., the non-uniformity of the hologram in FIG. 4(a) is depicted. It can be seen that as Z increases, the grating constant decreases. The general relationship between the grating constant and Z is as follows: EQU .LAMBDA.=.LAMBDA.(Z).
The absence of chromatic dispersion from Lippman holographic mirrors makes them highly desirable. Demand for large quantities of these holograms is foreseeable. State of the art production techniques for these holograms is insufficient to meet this demand for a number of reasons. In particular, the current apparatuses and methods for exposing holographic material to form a hologram are not satisfactory. There exists currently no apparatus or method to turn out large volumes of such holograms in a continuous fashion. Recording such holograms by the conventional prism method, where the sidewalls of a prism are exposed to laser light which illuminates the holographic medium at the bottom of the prism, is simply not at this time capable of recording holograms on a mass scale and imposes other important limitations on recording. Thus, it is evident that unless a new mass production method and apparatus is put to use, the increasingly high demand for high performance Lippman holographic mirrors will not be met.